http://blogs.discovermagazine.com/badastronomy/2009/06/24/the-blue-and-the-green/
This is one of the more compelling optical illusions I’ve seen. It really looks like there are 3 different colors in there.
Actually, while I’m at it, here’s another cool trick of the mind:
The guy who writes the Bad Astronomy blog - Phil Plait - used to post here.
You mean it looks like there are 4 different colors, right?
There are 3: magenta, orange (the stripes), and the bluish green used for the spirals.
Anyway, very cool.
I’m really starting to think that the folks who are paying (like I used to) and not seeing these ads are missing out on half the fun of these threads.
WTF is the above ad for? Every bit as entertaining as the two links in the OP however. 
There’s a black and white variation of this illusion. (Scroll down to #18)
Yeah. That confused me so much that I didn’t figure out what the illusion was supposed to be like I usually do. I should have guessed that the “blue” and “green”, since they appeared as variations of the same color, were actually the same color.
Incidentally, I wish the Gimp (or even Photoshop) had a perceptive colors option, so that the color would automatically change depending on what colors were around it. Instead, I’ll have to play around to figure out what color those appear to be.
That’s the one I’m familiar with. That’s why I’m surprised I didn’t recognize this one.
I meant three different colored spirals. My wording was unclear.
I believe the ads are different for everyone.
What… the…?
I don’t have words.
That one is cooler than the first one!
That’s very cool.
I really like this one: http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html
Can someone explain (using small words) how that castle illusion works? It’s truly amazing.
The first image is a blurred negative of the color image. When you stare at it, you fatigue the color receptors in your eye. When you move the mouse onto the image, it is replaced by a B&W version of it. You then get an afterimage (which is the negative of the first color image) superimposed on top of it. Since the negative of the first image is the correct colors, it looks like a full color version.
The color illusions drive me nuts. Most illusions I can see through or manipulate once I understand the illusion; I can force myself to see ‘curved’ lines as straight, or flip the cube facing at will, for example. But I cannot make myself see the ‘green’ and ‘blue’, or the ‘light grey’ and ‘dark grey’, as the same colors. When I sample the colors individually, I can see that they’re the same, but in context it’s impossible to bull through the illusion. That makes them special IMO.
Holy crap. At first I had no clue what anyone was talking about–nothing changed for me when I moved the mouse over. Then I switched from Mozilla to IE, and… holy crap.
I know, hey? I was all, “nice joke - it *is *in color. They just put the color picture in the mouseover to fool with people.” Then totally did a double-take when my eyes moved and I saw it was in b&w all along.
I doubt the browser makes a difference. I saw the color illusion just fine in Firefox. Of course, YMMV.
Running NoScript? When a page doesn’t seem to work right, that’s always the first thing I check. I allowed the domain and it worked fine.
On this particular one, if you look at the very edge of the picture the colors seem much more similar.
No the ads are the same, they just look different.